multiple external defs (Re: Lattice/UNIX incompatibility)

[Sam Kendall]

> 	3. Common sense indicates that the definition of
> 	an external variable should appear only once in the
> 	program text, so that its type can be changed by
> 	only altering one thing.  The logical place
> 	for such a single definition is in an include file.
> 	However, under the restrictive definition of C,
> 	this is impossible: the program breaks whether the
> 	include file says "extern" or not.
>
>	[ -- Andrew Koenig ]

On the other hand, one very good programming style dictates that the
declaration of an external variable, or any external name, appear in
two places: once in the specification of a module (in C, roughly
corresponding to a .h file) and once in the implementation of a module
(in C, a .c file); and perhaps even a third time, in the documentation
of the module.  This duplication ensures that the specification and
implementation remain compatible, since if only one changes the
compiler will complain; it encourages thinking about a program as a set
of modules with interfaces, rather than as a set of external variables;
and, of course, this duplication properly discourages external
variables by making them a pain in the butt to add (hee hee).

	Sam Kendall	  {allegra,ihnp4,ima,amd}!wjh12!kendall
	Delft Consulting Corp.	    decvax!genrad!wjh12!kendall